1. Field of the Invention
The invention relates to a method for operating an internal combustion engine, in particular with direct injection, in which a first fuel pump delivers fuel from a fuel tank to a second fuel pump that delivers fuel to a fuel accumulation line, and in which a metering unit is triggered by a control and/or regulating unit and meters the fuel quantity traveling into the inlet of the second fuel pump.
2. Description of the Prior Art
A method of operating an internal combustion is known from DE 199 26 308 A1, which discloses a pump apparatus for fuel, which has a main delivery pump embodied as a high-pressure pump, preceded by a presupply pump. The presupply pump is embodied as a mechanical fuel pump and delivers a fuel flow from a tank via a fuel line. The total fuel flow delivered is conveyed through the drive/crank chamber of the main delivery pump. Downstream of the drive/crank chamber, the total fuel flow is divided into a lubricating flow and a delivery flow. The lubricating flow travels via a return back to the tank. The delivery flow travels via a delivery circuit first to a metering unit and then on to the main delivery pump.
The main delivery pump is a radial piston pump driven by a camshaft. The radial piston pump feeds into a fuel accumulation line, which is also commonly referred to as a xe2x80x9crailxe2x80x9d. From the fuel accumulation line, the fuel travels to injection valves, which supply the fuel to combustion chambers of the engine.
It in order to be able to assure a reliable operation of injection valves, the pressure in the fuel accumulation line must not exceed a particular value. To this end, a pressure control valve is provided in the fuel accumulation line. If the fuel accumulation line is supplied with more fuel than is drawn by the injection valves, then above a particular pressure in the fuel accumulation line, this pressure control valve conveys the excess fuel out of the fuel accumulation line and returns it to the fuel tank.
One instance in which the pressure control valve is active, for example, is when the engine is being overrun while the control unit is simultaneously xe2x80x9cdeadxe2x80x9d. Here and in the following, a xe2x80x9cdeadxe2x80x9d control unit is understood to mean that it is no longer possible for the control unit to adjust the fuel quantity traveling to the main delivery pump. This can be the case both in the event of an electrical failure of the control unit and in the event of a mechanically jammed metering unit. During such an overrunning, generally no fuel whatsoever is injected by the fuel injection valves into the combustion chambers of the engine. Without a pressure control valve, when the control unit is xe2x80x9cdeadxe2x80x9d, the pressure in the fuel accumulation line could increase sharply and lead, for example, to an unwanted entry of fuel into the combustion chambers.
The object of the current invention is to modify a method of the type mentioned above so that the correspondingly operated internal combustion engine and here in particular, the fuel system of the engine, can be more simply designed and produced at a lower cost.
This object is attained with a method of the type mentioned above in that during normal operation, the metering unit is closed when it is without power and that when the control and/or regulating unit is xe2x80x9cdeadxe2x80x9d, the metering unit is without power and the fuel delivery to the metering unit is cut off.
The method according to the invention has the advantage that the internal combustion engine operated with it no longer requires a pressure control valve in the fuel accumulation line. Eliminating the pressure relief valve or pressure control valve in the fuel accumulation line simplifies the design of the engine and allows it to be produced at a lower cost.
An excessive increase of pressure in the fuel accumulation line and a resulting unwanted entry of fuel into the combustion chambers, for example during overrunning of the engine, is instead prevented by the method according to the invention by virtue of the fact that the fuel supply to the fuel accumulation line is reliably cut off as soon as a malfunction occurs in the control and/or regulating unit or in the metering unit. Since the powerless state of the metering unit is also its closed state, then this alone provides a highly reliable assurance that no more fuel travels to the second fuel pump. In addition, the fuel delivery to the metering unit is also cut off. This prevents the occurrence of any further delivery by means of the second fuel pump, even when the metering unit is in fact without power, but is mechanically jammed in the open state. This is based on the consideration that the pressure drop upstream of the second fuel pump and the opening pressure of the spring-loaded intake valves, which are generally used in the second fuel pump, cause this fuel pump to no be longer filled.
Another advantage of eliminating the pressure control valve lies in the fact that it is difficult or impossible to test its operational readiness. To be precise, a spring-loaded ball valve is usually used as the pressure relief valve or pressure control valve. But if the operational readiness cannot be regularly tested by the system, for example by means of a self test, then there can be no assurance in each case that the maximal permissible pressure in the fuel accumulation line will not be exceeded all the same.
With the method according to the invention, however, the operational readiness can be tested at any time. An interruption in the power supply to the metering unit, just like a discontinuation of the fuel delivery to the metering unit can be detected at any time by appropriate sensors. Consequently, the method according to the invention also increases the operational reliability of an internal combustion engine.
For example, the invention proposes that the first fuel pump be switched off when the control and/or regulating unit is xe2x80x9cdeadxe2x80x9d. This is easy to accomplish. Furthermore, an interruption of the power supply to the first fuel pump can be easily tested.
It is also possible to cut off the fuel supply to the first fuel pump by means of a shutoff valve device when the control and/or regulating unit is xe2x80x9cdeadxe2x80x9d.
This measure is particularly suitable if the first fuel pump cannot easily be switched off. This is the case, for example, with a first fuel pump that is mechanically driven, i.e. driven directly by the engine. An additional component is in fact required to execute this method, but its reliable operation can be tested at any time during the operation of the engine.
It is also advantageous to decouple a drive unit of the first fuel pump from the first fuel pump when the control and/or regulating unit is xe2x80x9cdeadxe2x80x9d. In particular, with a mechanically driven fuel pump, a clutch could be provided, which could decouple the drive unit from the pump as needed.
The invention also relates to a computer program, which is suitable for executing the method mentioned above, when it is run on a computer. It is particularly preferable if the computer program is stored in a memory, in particular a flash memory.
The invention also relates to a control and/or regulating unit for controlling and/or regulating at least one function of an internal combustion engine. With a control and/or regulating unit of this kind, it is advantageous if it is provided with a computer program of the type mentioned above.
The invention also relates to a fuel system for an internal combustion engine, in particular with direct injection, having a fuel tank, a first fuel pump that delivers from the fuel tank, and a second fuel pump, which is connected on the inlet side to the first fuel pump and is connected on the outlet side to a fuel accumulation line, and having a metering unit that meters the fuel quantity traveling into the inlet of the second fuel pump.
A fuel system of this kind is also known from DE 199 26 308 A1 and has already been explained above. In order to be able to design a fuel system of this kind more simply and produce it at a lower cost, the invention proposes that the first fuel pump be electrically driven and that the metering unit be closed when it is without power.
An electrically driven first fuel pump can be shut off in a simple manner: in particular, a malfunction of a control and/or regulating unit that controls and/or regulates the fuel system or a general power failure automatically causes the first fuel pump to be shut off and this alone cuts off the delivery in the direction of the fuel accumulation line. In order to achieve a redundancy, the metering unit is designed so that it is closed when it is without power.
Alternatively, it is possible to drive the first fuel pump mechanically and to provide a shutoff valve device between the fuel tank and the first fuel pump. This has the advantage that a very simply designed and rugged fuel pump can be used and the shutoff valve device can still cut off the fuel supply in the direction of the fuel accumulation line, for example when the engine is being overrun and a malfunction simultaneously occurs in the operation of the engine.
It is particularly advantageous if the shutoff valve device is closed when it is without power. Primarily in the event of a failure of a control and/or regulating unit, this reliably cuts off the fuel supply to the fuel accumulation line. A fuel system of this kind operates redundantly if the metering unit is also closed when it is without power.
In a preferred modification of a fuel system of the type mentioned above, the outlet side of the first fuel pump is initially connected to a drive/crank chamber of the second fuel pump and the outlet side of the drive/crank chamber of the second fuel pump is connected to an overflow line that contains an overflow valve, which adjusts the pressure in the drive/crank chamber to a particular value. This modification is based on the following concept:
When the drive means in the drive/crank chamber rotates or moves, a large amount of thermal and/or mechanical stress is generated in this region. The high fuel flow according to the invention in the vicinity of the drive/crank chamber makes a particularly good lubrication and heat dissipation possible. In particular, a positive lubrication of the drive/crank chamber is possible since the drive/crank chamber is subjected to the total fuel flow with the full delivery pressure of the first fuel pump. The fuel system according to the invention is therefore distinguished by a long service life and a reliable operation, without requiring an additional coolant circuit and/or lubricant circuit.
A particularly advantageous modification of a fuel system is the one in which a zero-delivery line branches off between the metering unit and the second fuel pump, is connected to the fuel tank or the inlet of the first fuel pump, and carries away an overflow that occurs when the metering unit is closed. Such a zero-delivery line assures that when a residual quantity of fuel nevertheless is still emerging from the outlet of the metering unit, this quantity is not forced into the fuel accumulation line, but travels back into the fuel tank via the zero-delivery line.
To that end, the opening pressure of possibly provided intake valves of the second fuel pump should be selected (preferably greater than 2 bar) so that the pressure drop due to the zero-delivery line does not cause the intake valves to open.
Placing a zero-delivery throttle in the zero-delivery line assures that during normal operation of the fuel system, i.e. when the second fuel pump is delivering fuel in the direction of the fuel accumulation line, as little fuel is possible travels back to the fuel tank through the zero-delivery line.
The metering unit can be produced in a particularly inexpensive matter if it includes an electric sliding valve. The same is true for a shutoff valve unit, which includes a magnetic on-off valve.
The fuel system according to the invention permits a pressure control valve to be eliminated. This is explicitly expressed in a modification of the fuel system according to the invention.